Three times my Prius has had cracks in the windshield from rocks hitting it. This time I replaced the windshield instead of having them plug the crack. I asked the glass guy if he gets a lot of Priui. He said he did. He gave some theory why but I said I had a different theory:
When the windshield is more vertical air pressure builds in front of the windshield and protects it. Any small rock heading toward a vertical windshield is slowed by this layer of presserized air. The Prius’ windshield is more horizontal than vertical so the air pressure is much less (better gas mileage) so rocks hit without any buffer.